1. Technical Field
The invention relates to photonics. In particular, the invention relates to metamaterial inclusion structures that provide a magnetic response optical frequencies.
2. Description of Related Art
A metamaterial is a composite material that derives a material property from a combination of its composition and its structure rather than exclusively from its bulk composition. In particular, metamaterials are manmade materials that generally comprise arrays of inclusion structures. The inclusion structures, which are usually much smaller than a wavelength of an excitation signal, act together to produce an aggregate response (or material response) to the excitation signal. For example, metamaterials that exhibit a negative index of refraction (so-called negative index materials (NIMs)), a material property that is not available in natural materials, have been demonstrated. Such metamaterials may be realized by combining a material with negative permittivity ∈ and a material with negative permeability μ, for example. Metamaterials have a number of intriguing real-world applications including, but not limited to, producing a so-called superlens which may provide resolutions that exceed a diffraction limit at an operational wavelength and even “cloaking devices” that could make an object essentially invisible to incident electromagnetic radiation.
Metamaterials in both the microwave and optical domains have been demonstrated beginning with work by W. E. Kock in the 1940's. Kock developed metal lens antennas and metallic delay lines that, while not described at the time as such, essentially comprised metamaterials. Note that the term ‘metamaterial’ was first coined in 1999 by R. M. Walser and has been used only more recently to describe composite materials including, but not limited to, those developed by Kock and others prior to the 1990's.
Optical metamaterials have also been demonstrated. Optical metamaterials may be realized by constructing an array of inclusion structures with sub-wavelength dimensions that exhibit a response (i.e., resonance) to one or both of an electric field component and a magnetic field component of an optical excitation signal. A number of examples of optical metamaterials comprising inclusion structures that exhibit relatively strong electric field responses have been reported by N. Engheta, N. Liu et al. and others.
However, producing a strong (or even a weak) response to a magnetic field component of the incident optical signal is more difficult. In particular, unlike the case in the radio frequency (RF) domain, producing a magnetic response with a metamaterial inclusion structure at optical wavelengths is problematic due to various specific characteristics of metals typically used to realize such structures. As such, there is interest in finding means for realizing metamaterial inclusion structures that exhibit a magnetic response or resonance in the optical domain. Such a metamaterial inclusion structure with optical domain magnetic responses may facilitate the production of metamaterials with negative effective permeability, and further, negative indices of refraction at optical frequencies that would satisfy a long felt need.